1. Field of the Invention
The present invention relates to a control mechanism for initiating a defrost cycle associated with a refrigeration circuit having a heat exchanger or other heat transfer element on which frost may form. More specifically, the present invention concerns a control device for varying the time between defrost cycles as a function of the length of the previous defrost cycle.
2. Description of the Prior Art
Air conditioners, refrigerators and heat pumps produce a controlled heat transfer by the evaporation in an evaporator chamber of a liquid refrigerant under pressure conditions which produce the desired evaporation temperatures. The liquid refrigerant absorbs its latent heat of vaporization from the medium being cooled and in this process is converted into a vapor at the same pressure and temperature. This vapor has its temperature and pressured increased by a compressor and is then conveyed into a condenser chamber in which the pressure is maintained at a predetermined level to condense the refrigerant at a desired temperature. The quantity of heat removed from a refrigerant in the condenser is the latent heat of condensation plus the super heat which has been added to the liquid refrigerant in the process of conveying the refrigerant from the evaporator pressure level to the condenser pressure level. After condensing, the liquid refrigerant is passed from the condenser through a suitable throttling device back to the evaporator to repeat the cycle.
In a closed cycle system, generally a mechanical compressor or pump is used to transfer the refrigerant vapor from the evaporator (low pressure side) to the condenser (high pressure side). The vaporized refrigerant drawn from the evaporator is compressed and delivered to the condenser wherein it undergoes a change in state from a gas to a liquid transferring heat energy to the condenser cooling medium. The liquefied refrigerant is then collected in the bottom of the condenser or in a separate receiver and fed back to the evaporator through the throttling device.
Evaporators of many different types are known in the art and all such evaporators are designed with the primary objective of affording easy transfer of heat from the medium being cooled to the evaporating refrigerant. In one commonly known type of evaporating system (direct expansion), refrigerant is introduced into the evaporator through a thermal expansion valve and makes a single pass in thermal contact with the evaporator surface prior to passing into the compressor suction line.
While the evaporator functions to collect refrigerant to pass from a liquid state into a vapor state extracting the latent heat of vaporization of the refrigerant from the surrounding medium, the function of the condenser is the reverse of the evaporator, i.e. to rapidly transfer heat from the condensing refrigerant to the surrounding medium. One of the frequently encountered well-known problems associated with air source heat pump equipment is that during heating operations the outdoor coil which is functioning as an evaporator tends to accumulate frost which reduces the efficiency of the system. In order to periodically remove the accumulated frost, various defrosting systems have been devised such as heating the coils or reversing the operation of the system. However, whatever the particular defrosting system employed in the heat pump, it is necessary for the optimum system efficiency to determine when the outdoor coil should be defrosted.
The accumulation of frost on the heat exchange surfaces of the evaporator produces an insulating effect which reduces the heat transfer between the refrigerant flowing through the evaporator and the surrounding medium. Consequently, after a buildup of frost on the heat exchanger heat transfer surfaces the heat pump system will lose capacity and the entire system will operate less efficiently.
In order to obtain maximum system efficiency, it is desirable to select the optimum time-to-initiate defrost such that the heat pump system is not operated during those periods when there is sufficient frost buildup to substantially interfere with heat transfer between the refrigerant flowing through the evaporator and the surrounding medium. It is also desirable, however, to provide a minimum number of defrost cycles since each defrost cycle may result in removing heat from the enclosure to be conditioned, energizing electric resistance heaters, or reversing refrigeration systems such that heat normally supplied to the space to be conditioned is used to defrost the evaporator. Each defrost cycle detracts from the overall efficient performance of the heat pump system. Consequently, it is important to strike a balance between initiating defrost before heat transfer is substantially diminished by frost accretion and preventing the rapid cycling of the system between defrost and heating operations. This frost buildup situation is not only related to the evaporator of a heat pump but it finds like applicabilty in other cold applications wherein the evaporator is operated at a temperature below the freezing point of moisture in a surrounding medium such as a freezer compartment, a refrigerator, cold storage rooms, trailer refrigeration equipment, humidifiers, and supermarket display cases.
Different types of frost control systems have been utilized, varying from the use of the timer to periodically initiate and terminate defrost to sophisticated infrared radiation and sensing means mounted on the fins of the refrigerant carrying coils. Other such defrost systems generate a signal in response to an air pressure differential across the heat exchanger caused by frost accumulation blocking the airflow through the heat exchanger. Other defrost systems require coincidence between two independently operable variables each of which may indicate frost accumulation such as air pressure within the shroud of the evaporator and the temperature differential within the evaporator coil. Another system may be the combination of a periodic timer to initiate defrost with a thermostat for sensing refrigerant temperature to terminate defrost. Another defrost system is one wherein compressor current or another operational parameter is monitored and compared to a reference level signal developed during a non-frost condition such that a variation from that reference level of the parameter being monitored indicates that it is time-to-initiate the defrost cycle.
These defrost systems can generally be grouped into two specific categories: timed and demand. A timed system simply initiates defrost periodically whether frost has accumulated or not based on the knowledge that all heat pump systems will need periodic defrosting under certain weather conditions. The amount of time chosen for periodically initiating defrost is a compromise between a short time that would cause a waste of efficiency during weather conditions which do not necessitate defrost and a long time which would allow the heat pump to operate inefficiently with a severely frosted evaporator coil. The advantage of a timed defrost system is that the heat pump will be defrosted periodically. The disadvantage is that the needed time between defrosts is never quite the same as the preset time due to weather conditions which differ from day to day and location to location.
Demand defrost systems attempt to initiate a defrost cycle as a function of some system parameter which is related to a measure of frost accumulation. The advantage of a demand defrost system is that the heat pump is allowed to continue normal operation without energy consuming defrost cycle until defrost is actually required. The disadvantage of demand defrost systems is that initial equipment cost is high and demand systems are less reliable in their ability to sense the need for defrost.
The herein disclosed defrost control mechanism is a combination of timed and demand. The parameter being monitored is the elapsed time during a previous defrost cycle. The interval between defrost cycles is a continually changing time as a function of the time in defrost.